Au-Ag Pre

Assessment of Ni/Pd/Au-Pd and Ni/Pd/Au-Ag Pre-plated Leadframe Packages Subject to Electrochemical Migration and Mixed Flowing Gas Tests Ping Zhao and Michael Pecht CALCE Electronic Products and Systems Center Department of Mechanical Engineering University of Maryland, College Park, MD 20742 USA Sungil Kang and Sechul Park Samsung Techwin Changwon, Kyoungnam, South Korea

Abstract: Pre-plated leadframes packages with a new noble metal finish type were exposed to various electrochemical migration tests, and mixed flowing gas environments to assess the quality and reliability due to these environmental conditions. Packages were tested as discrete parts and also as parts mounted onto printed circuit boards to assess the effect of reflow soldering. Selected packages with SnPb plated leadframes were tested for comparison purposes. After test exposure, optical microscopy, SEM and EDS analyses were conducted to evaluate the corrosion behavior. Corrosion and creep corrosion on the leads were observed on all the pre-plated test samples subject to the mixed flowing gas. Creep corrosion over the mold compound surface was identified on the un-mounted QFN packages. The creep corrosion products were found to be electrically conductive and could bridge adjacent lead pairs.

Keywords: Pre-plated Leadframe, Mixed Flowing Gas, Electrochemical Migration, Creep Corrosion

1. Introduction For electronic components produced with leadframes, it is necessary to protect the exposed leads from oxidation and corrosion, to help obtain high quality solderability for component assembly onto a printed wiring board (PWB) and a resultant reliable solder joint [1][2]. The normal approach is to coat the leadframe with a protecting finish or plating.

Leadframe finishing can be accomplished by pre- or post-encapsulation processes. In the postencapsulation process, generally a coating of tin-lead or some other tin-based alloy is applied by hot

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dipping or electroplating after mold compound encapsulation 1 . In the pre-encapsulation process, noble metals are generally plated over the entire leadframe prior to die bonding, wire bonding and encapsulation.

Using palladium as a plating material for electronic products was first proposed in the mid 1970’s for contacts and connectors [5]. An electrolytic plating of pure palladium over nickel was found to provide corrosion resistance and wear properties similar to that of hard gold. The success of palladium-plated contacts and connectors eventually led to its consideration for replacement of solder finishing of leadframes in the semiconductor industry. Texas Instruments (TI) first introduced the palladium-over-nickel leadframe technology as a pre-encapsulation practice for their small outline, low pin-count integrated circuit (SOIC) devices in 1989 [6]. By 1999, TI reported that it had fielded more than 30 billion IC packages with this type of pre-plating finish [7], including fine-pitch (0.64mm or less) high pin-out surface-mount packages.

Pre-plated leadframes have many advantages over conventional solder-coated leadframes [7]. The preplated leadframe eliminates the solder coating process in manufacturing, and can reduce cycle time and overall production costs in semiconductor assembly. Lead in solder dipping and cyanide in silver spot plating process are removed from the process, eliminating health and safety concerns associated with exposure to these hazardous materials in the work place and on products. Noble metal pre-plate also avoids issues associated with lead and cyanide disposal. Component quality has been reported to be improved due to higher IC planarity and lower possibility of solder bridging, compared to post-plated methods [8]-[12]. In addition, tin whiskering has not been observed on such components. TI’s self-qualification studies [8][12] have suggested that the palladium lead finish provides at least equivalent protection capability, solderability, and product shelf life, compared to that of the standard tin-lead solder finishes.

The disadvantages of pre-plated leadframes include the high cost of the noble metals, concerns with visual inspection criterion of the solder joint, and reliability. Benedetto [13] suggests that palladium lead finish components cannot be utilized as a drop-in replacement for SnPb coated components based on their

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Due to the up-coming European lead-free legislation, any coating material containing lead will be prohibited, with some minor exemptions [3]. As a result, pure tin coating has gained wide industry support as a lead finish, in spite of its potential for tinwhiskering [4]

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solderability and reliability test results. Furthermore, when components are subjected to corrosive environments, corrosion may initiate from exposed base metal in the dam-bar removal, cracks at lead bend, and surface porosity [14]. Corrosion products can induce storage problems if the solderability of the leads degrades. Reliability problems can occur if corrosion products move over the leadframe and onto the mold compound surface and electrically bridge adjacent leads. This phenomenon is known as creep corrosion [14] -[18].

The first pre-plated leadframe finish included 4-layer Ni/Pd plating (Ni strike/Ni-Pd strike/Ni/Pd) and 2layer Ni/Pd plating (for both cases, Ni/Pd nominal thickness of 1.27/0.075 microns) [7]. However, in late 1990s, due to the dramatic increase in the price of palladium, the plating thickness of the palladium was reduced and a gold flash was added over the palladium. Now, TI has chosen Ni/Pd/Au finish (nominal thickness values of 0.5/0.02/0.003 microns) as the lead-free finish for all their leadframe-based packages [19]. Today, all TI logic IC devices are available in pre-plated leadframe packages [19], and many other companies (e.g. Samsung, Sony, and Freescale) also have noble metals pre-plated leadframe based components.

Samsung Techwin Co. Ltd. initially provided leadframes using Ni/Pd/Au-Pd as the leadframe finish (nominal thickness values of 0.5/0.013/0.005 microns), and then changed to Ni/Pd/Au-Ag as the finish material (nominal thickness values of 0.5/0.013/0.005 microns); both of which have been commercially marketed as µ-PPF. The addition of Ag to the Samsung Techwin plating metallurgy raises concerns of Ag migration, because Ag-based materials have the potentials to migrate in humid and corrosive environments, sometimes forming dendrites under electrical bias [20][21]. Although the Ag on the pre-plated leadframes will significantly diffuse into the bulk solder upon reflow, testing is necessary to evaluate the possibility of corrosion, silver migration and other reliability concerns, before and after reflow.

2. Experiments This paper describes two sets of tests that were conducted on µ-PPF packages with both the Ni/Pd/Au-Pd and Ni/Pd/Au-Ag finishes. The tests were electrochemical migration and mixed flowing gas.

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2.1 Electrochemical Migration Tests Electrochemical migration is defined in IPC-9201 as “the growth of conductive metal filaments on a printed wiring board under the influence of a DC voltage bias” [22]. This may occur at an external surface, an internal interface, or through the bulk material of a composite. Figure 1 shows the diagram of some possible electrochemical migration failures on a mounted integrated circuit package.

Electro-chemical migration products

Mold compound

Package lead

Figure 1: Diagram of electrochemical migration on the mounted package

The test standard for electrochemical migration testing is IPC-TM-650 method 2.6.14.1 [23]. Table 1 lists the test conditions and Table 2 lists the samples tested in the study. During the environmental exposure, the surface insulation resistance between adjacent lead pairs were continuously monitored and recorded. A drop of insulation resistance below 108 ohms is considered an indication of electrical leakage current. After exposure and test, visual inspection was conducted to evaluate evidence of electrochemical migration. Table 1: Conditions for electrochemical migration tests Temperature

R.H.

Electrical bias

Duration

Condition 1

40ºC

93%

10V DC

168 hours

Condition 2

65ºC

88.5%

10V DC

168 hours

Condition 3

85ºC

88.5%

10V DC

168 hours

Condition 4

40ºC

93%

10V DC

500 hours

4

Table 2: Samples for electrochemical migration tests Package

Pitch

Leadframe base metal

Leadframe finish

Quantity

TSSOP56

0.5mm

C7025

Ni/Pd/Au-Ag

3

TSSOP56

0.5mm

C7025

Ni/Pd/Au-Pd

3

QFN48

0.5mm

C194

Ni/Pd/Au-Ag

3

QFN48

0.5mm

C194

SnPb

3

2.2 MFG Test Accelerated testing using mixed flowing gases (MFG) [24] is often used to evaluate and assess the corrosion behavior of packages with noble metal pre-plated leadframes. The MFG test is conducted in a test chamber in which the temperature, relative humidity, and concentration of selected gases are carefully controlled. In our study, test samples were subjected to 15 days of Battelle Class III MFG environment, having test conditions listed in Table 3 [25]. This test is intended to simulate industrial locations, where moderate amounts of pollutants are present in poorly controlled environments. Table 3: Battelle Class III MFG test conditions [25] Class

Temp (ºC)

RH (%)

H2S (ppb)

Cl2 (ppb)

NO2 (ppb)

III

30±2

75±2

100±10

20±5

200±25

Table 4 lists the samples tested in this study. The experiment included un-mounted and assembled components, with different package styles, different base metals, and different plating structures. Two preplating compositions, Ni/Pd/Au-Pd and Ni/Pd/Au-Ag, were investigated. For Ni/Pd/Au-Pd plating, a nominal thickness of 0.013 microns of Pd is plated over a nominal thickness of 0.5 microns of Ni. A nominal thickness of 0.005 microns of Au-Pd alloy is plated over Pd as the surface flash. For Ni/Pd/Au-Ag plating, all the nominal values are the same, but the surface flash is an Au-Ag alloy.

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Table 4: Samples tested for mixed flowing gas Type

Test Status

Package

Base metal

Plating

Quantity

1

Un-mounted component

SOP32

C194

Ni/Pd/Au-Ag

13

2

Un-mounted component*

SOP32

C194

Ni/Pd/Au-Ag

13

3


SOP32

C194

Ni/Pd/Au-Pd

13

4

Un-mounted component*

SOP32

C194

Ni/Pd/Au-Pd

13

5


TSSOP56

C7025

Ni/Pd/Au-Ag

13

6


TSSOP56

C7025

Ni/Pd/Au-Pd

13

7


QFN48

C194

Ni/Pd/Au-Ag

13

8


QFN48

C194

SnPb

13

9


TSOP66

Alloy42

Ni/Pd/Au-Ag

13

10

Component on PCB

TSSOP56

C7025

Ni/Pd/Au-Ag

1

11

Component on PCB

TSSOP56

C7025

Ni/Pd/Au-Pd

1

12

Component on PCB

QFN48

C194

Ni/Pd/Au-Ag

1

13

Component on PCB

QFN48

C194

SnPb

1

14

Component on PCB

TSOP66

Alloy42

Ni/Pd/Au-Ag

1

* Those un-mounted components were also subject to a reflow profile to simulate the soldering process. For all un-mounted components (type 1 through 9 listed in Table 4), nylon line was used to suspend the number “1” lead of each component to a wood rod, and then the rod was placed on a rack inside the MFG chamber. The quad flat no-lead (QFN) components were tied around the center of the package. A set of samples were taken out after 12-hr, 1-day, 2, 3, 4, 5, 6, 7, 8, 9, 10-day exposures were evaluated, but were not put back into the chamber. After 15 days of exposure, the test was terminated and the last two sets of samples were removed from the chamber for evaluation.

For board mounted test samples (types 10 through 14 listed in Table 3), one board for each type was tested. These boards were suspended by nylon line inside the chamber. After 7 and 10 days of exposure, the

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boards were taken out of test chamber for evaluation and then put back into the chamber for continued exposure.

Photos were taken under the optical microscope to assess the changes to the surface of the component leads. Environmental scanning electron microscopy (ESEM) and energy dispersive spectroscopy (EDS) analysis was conducted to assess the corrosion on the leads and creep corrosion products on the mold surface.

3. Results and Discussion In the electrochemical migration test, regardless of the package type, plating materials and pitch size, there was negligible surface insulation resistance drop observed at the environmental conditions specified in the IPC test standards (40°C /93% R.H. for 168 hrs and 500 hrs, 65°C /88.5% R.H. and 85°C /88.5 % R.H. for 168 hrs). For instance, examples are shown in Figure 2 and Figure 3. Optical examination of the samples after these exposures confirmed the results. This implies that the tested samples meet or exceed the IPC test standard for board level testing for electrochemical migration. 1.E+13 1.E+12

Resistance (Ohm)

1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 0

50

100

150

200

Time (hours) Figure 2: Insulation resistance change on board mounted TSSOP56 with Ni/Pd/Au-Ag plating during 168 hours of exposure in 85ºC/88.5% R.H.

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1.E+13 1.E+12

Resistance (Ohm)

1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 0

50

100

150

200

Time (hours)

Figure 3: Insulation resistance change on board mounted TSSOP56 with Ni/Pd/Au-Pd plating during 168 hours of exposure in 85ºC/88.5% R.H. In the MFG tests, corrosion was observed on the leads of all tested components. Figure 4 shows the evolution of corrosion over leads on the SOP 32 with Ni/Pd/Au-Ag finish (type 1). Prior to exposure, the plating was shiny and free from visible corrosion products. After 12 hours of exposure, the luster of plating was blurred by corrosion products appearing on the surface. With extended MFG exposure, the whole surface of plating was covered by corrosion products, which was the effect of creep corrosion. This is not unusual for pre-plated leadframe components because the dam-bar opening is a significant source for corrosion [14]. Type 2 components behaved the similar way as the type 1, which showed that additional reflow did not exhibit a significant impact on the corrosion appearance for this MFG test.

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12 hrs

No exposure

5 days

15 days

Figure 4: Evolution of corrosion over leads on the SOP 32 with Ni/Pd/Au-Ag finish (type 1) in mixed flowing gas test The board mounted samples showed similar corrosion appearance to that of the un-mounted samples (see Figure 5 and Figure 6). It was demonstrated that the component assembly process, including reflow and solder joint forming, did not inhibit corrosion in the MFG test conditions. This is also consistent with existing findings on Ni/Pd and Ni/Pd/Au pre-plated samples [18].

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Figure 5: Corrosion over leads on un-mounted TSSOP56 with Ni/Pd/Au-Pd finish (type 6) after 15 days of MFG exposure

Figure 6: Corrosion over leads on mounted TSSOP56 with Ni/Pd/Au-Pd finish (type 11) after 15 days of MFG exposure

Corrosion and creep corrosion on the leads were observed on all pre-plated components. Figure 7 and Figure 8 show the corrosion over leads on components with Cu and Alloy 42 base metal materials, in which

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base metal materials make a difference in the corrosion product color and morphology. The two plating structures, Ni/Pd/Au-Pd and Ni/Pd/Au-Ag, showed similar corrosion appearance. This is expected, since the corrosion product is independent of the pre-plated noble metals [18].

Figure 7: Corrosion over leads on un-mounted TSSOP56 with Ni/Pd/Au-Ag finish (type 5) after 15 days of MFG exposure

Figure 8: Corrosion over leads on un-mounted TSOP66 with Ni/Pd/Au-Pd (type 9) after 15 days of MFG exposure (black and brown materials are generated corrosion products on leads)

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On QFN packages, the SnPb coated samples exhibited much less corrosion at exposed pads and die paddle than those with the Ni/Pd/Au-Ag finish (see Figure 9 and Figure 10). It appeared that SnPb coating provides a better protection from environmental attack compared with noble metal pre-plating. This is in part because the SnPb coating is thicker, while the Au-Ag tends to be porous. In addition, corrosion of the base metal with the coating of SnPb is inhibited by the accumulated corrosion products.

Usually corrosion products on the lead surface appeared dense and dark in color. For some locations on the leads of un-mounted QFN packages, the corrosion products were observed to have a green porous morphology (see Figure 11). This may be due to the corrosion of bulk base metal (Cu) exposure on the package corner and side.

Figure 9: Overview of corrosion over un-mounted QFN with Ni/Pd/Au-Ag finish (type 7) after 15 days of MFG exposure

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Figure 10: Corrosion on un-mounted QFN with SnPb finish (type 8) after 15 days of MFG exposure

Figure 11: Porous corrosion products on un-mounted QFN with Ni/Pd/Au-Ag finish (type 7) after 15 days of MFG exposure

Creep corrosion over the molding compound surface was observed on the un-mounted QFN packages with the Ni/Pd/Au-Ag finish in as little as 8 days in the MFG environment (see Figure 12). After 15 days of

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exposure, noticeable creep corrosion products were identified as bridging leads (see Figure 13 and Figure 14). For the adjacent lead pairs with visible creep corrosion, the insulation resistance was measured to be around 6MΩs.

Figure 12: Creep corrosion over molding compound surface on QFN package with Ni/Pd/Au-Ag finish (type 7) after 8 days of MFG exposure

Figure 13: Creep corrosion over molding compound surface on QFN package with Ni/Pd/Au-Ag finish (type 7) after 15 days of MFG exposure

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Figure 14: ESEM view of creep corrosion over molding compound surface on QFN package with Ni/Pd/Au-Ag finish (type 7) after 15 days of MFG exposure

The EDS analysis of creep corrosion products on the molding compound surface shows elements of Cu, O, Cl, and S (see Figure 15), which are the same as the elements found on lead surface corrosion products.

Figure 15: EDS spectrum to show elements of Cu, S, Cl, and O in the creep corrosion products

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Figure 16 shows that the leading edge area of creep corrosion on the mold compound surface, which appears the dendritic formation. Dendrite is usually related with corrosion product formation under electric bias [26] or crystallization growth in solidification process [27]. In this case, no electrical bias is present on the molding compound and corrosion product progression is not a solidification process. It is believed that corrosion products dissolve into the surface adsorbed water for transportation and redeposition of ionic materials forms the dendritic growth pattern.

Figure 16: ESEM of the leading edge area of creep corrosion products over molding compound surface on QFN package with Ni/Pd/Au-Ag finish (type 7) after 15 days of MFG exposure

5. Conclusions For the four types of packages subject to the 3 types of conditions (40°C /93% R.H. for 168 hrs and 500 hrs, 65°C /88.5% R.H. and 85°C /88.5 % R.H. for 168 hrs) of electrochemical migration tests per IPC-TM-650 method 2.6.14.1, no electrochemical migration failures, in terms of insulation resistance drop, were observed. This implies the tested samples with µ-PPF exceeded the IPC test standards for board level electrochemical migration testing. This is an expected result for the mounted devices, because the finish significantly diffuses into the bulk solder during reflow.

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Corrosion and creep corrosion on the leadframe surface occurred on all test samples in the Battelle Class III environment. After 15 days, there were significant corrosion products on the leadframe. There was no significant difference between the corrosion appearance of the various package styles, base metal materials, and noble metal plating structures tested in the study. The SnPb plated QFN showed less corrosion than the QFN with the Ni/Pd/Au-Ag finish.

Creep corrosion on the mold surface was only observed on the QFN packages with the Ni/Pd/Au-Ag finish, and occurred as early as 8 days of exposure. Creep corrosion products on the mold compound had dendritic formations that could electrically bridge adjacent lead pairs. Using the acceleration factor provided by Williams [28], 8 days in the Battelle Class III environmental test is equivalent to about 4 years of field operation or storage in an industrial environment.

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[21] Dumoulin, P., Seurin, J.P., Marce, P., “Metal Migrations Outside the Package during Accelerated Life Tests,” IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 5, Issue 4, pp. 479-486, Dec. 1982. [22] IPC-9201, “Surface Insulation Resistance Handbook,” IPC, July 1990. [23] IPC-TM-650 method 2.6.14.1, “Electrochemical Migration Resistance Test,” IPC, Sep. 2000. [24] Abbott, W.H., “The development and performance characteristics of flowing mixed gas test environments,” IEEE Transactions on Components, Hybrids, and Manufacturing Technology, vol. 1l, no. 1, pp. 22-35, Mar. 1988. [25] ASTM B845-97, “Standard Guide for Mixed Flowing Gas (MFG) Tests for Electrical Contacts,” American Society for Testing and Material, 2003. [26] Zamanzadeh, M., Meilink, S.L., Warren, G.W., Wynblatt, P., and Yan, B., “Electrochemical Examination of Dendritic Growth on Electronic Devices in HCl Dlectrolytes,” Corrosion, vol. 46, no. 8, pp. 665-671, Aug. 1990. [27] Libbrecht, K.G., “The Physics of Snow Crystals,” Reports on Progress in Physics, vol. 68, no. 4, pp. 855-895, April 2005. [28] Williams, D.W., “The Effect of Test Environment on the Creep of Base Metal Surface Films over Precious Metal Inlays,” IEEE Transactions on Components, Hybrids and Manufacturing Technology, vol. 11, no. 1, pp. 36-42, Mar. 1988.

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